1. Field of the Invention
This invention relates generally to a crank shaft of a V-type internal-combustion engine and particularly to a 3-web, 2-pin crank shaft comprising two crank pins each connected via piston rods to plurality of pistons engaged slidably in a plurality of cylinders being arranged in a V shape, with three crank webs carrying the crank pins.
2. Description of the Prior Art
In a V-type multi-cylinder internal-combustion engine, the primary inertia force exerted by the mass of the reciprocating parts on the crankshaft can be cancelled out by offsetting the rotary phases of the two crank pins by an angle of .pi.-2.beta., where .beta. is the angle between the central axes of the cylinders arranged in the V shape. This construction, however, results in a couple of the forces due to the mass of the rotating parts such as the big ends of the connecting rods connected to each crank pin, causing undesirable vibrations in the engine.